High-speed spectrographic sensor for internal combustion engines

ABSTRACT

A high-speed absorption spectrographic system employs a slit-less spectroscope to obtain high-resolution, high-speed spectrographic data of combustion gases in an internal combustion engine allowing precise measurement of gas parameters including temperature and species concentration.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with United States government support awarded bythe following agencies: NSF 0238633.

The United States has certain rights in this invention.

CROSS-REFERENCE TO RELATED APPLICATIONS

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BACKGROUND OF THE INVENTION

The present invention relates to instruments for the study of combustiongases and in particular to an improved sensor system for providinghigh-speed optical measurements of combustion gas temperature, watermole fraction and the like.

Knowledge about combustion, including combustion temperature andcombustion gas composition, can be important in the study and control ofinternal combustion engines. For this purpose of measuring combustiongas temperature, it is generally known to use an optical pyrometerobserving light emitted from the combustion gases and/or materialscontacting combustion gases. For example, U.S. Pat. No. 6,370,486describes a sensor that measures infrared energy emitted at severalpreselected wavelengths from hot gas to calculate gas temperature.

A more sophisticated system is described in U.S. Pat. No. 6,640,199,which analyzes the emission spectrum of the combustion gases to deducetemperature and relative concentration of some chemical species makingup the combustion gas.

SUMMARY OF THE INVENTION

The present invention provides a measurement of combustion gastemperature and species concentration using absorption spectroscopytechniques. In contrast to the measurement of emission spectra, suchabsorption spectroscopy requires the introduction of a known lightsignal into the combustion space and the extraction of sufficient energyat multiple light frequencies to perform the spectroscopic measurement.The present invention meets these requirements while using a light guidethat may be as small as a fiber optic, by employing a sensing systemsthat eliminates the standard optical slit required of, for example,grating spectrometers. The elimination of the optical slit or similaraperture reducing structure improves the use of light energy and allowshigh-resolution spectrographs to be created at an extremely high rate.

Specifically, the present invention provides a high-speed spectrographicsensor for internal combustion engines having a plug receivable into acombustion chamber of an operating internal combustion engine and alight source providing light at multiple frequencies between 2000 and3000 nm. A light guide, for example one or more optical fibers held bythe plug, receives the light source to communicate the light into thecombustion chamber for interaction with combustion gases. The lightguide also communicates the light out of the combustion chamber forsensing. A sensor system distinguishes the strength of the light afterinteraction with the combustion gases at no less than twenty multiplefrequencies and at a rate of no less than 1000 times per second.

Thus it is an aspect of at least one embodiment of the invention toprovide real-time multi-spectral absorption measurements of combustiongases.

The sensor system may be a spatial heterodyne spectroscope receiving thelight from the light guide after the light has passed through thecombustion chamber.

Thus it is another aspect of at least one embodiment of the invention toprovide for a spectrographic decomposition that avoids the energy lossincident to a standard slit or similar-type spectrometer. The spatialheterodyne spectroscope may operate with an input aperture that is asmuch as two orders of magnitude larger than a slit spectroscope.

The sensor system may further include a computer deducing and outputtingtemperature within the combustion chamber from the strengths of themultiple frequencies.

It is thus another aspect of at least one embodiment of the invention toprovide for automatic temperature measurements of combustion gases.

The computer may deduce and output water concentration within thecombustion chamber from the strengths of the multiple frequencies.

It is thus another aspect of at least one embodiment of the invention toprovide for automatic measurements of water mole fractions.

The computer may deduce a physical parameter of combustion gases bymatching the strengths of the multiple frequencies to correspondingmultiple frequencies of signatures representing known different physicalparameters within the combustion chamber.

It is thus another aspect of at least one embodiment of the invention toallow complex analysis of absorption spectra on an automatic basis.

The light source may provide frequencies substantially within a range of2400-2600 nm.

It is thus another aspect of at least one embodiment of the invention toprovide for absorption measurements in a novel frequency band forcombustion gases.

The sensor system may distinguish the strength of no less than 100multiple frequencies.

It is thus another aspect of at least one embodiment of the invention toprovide for the measurement of high-resolution absorption spectra ofcombustion gases.

The sensor system may distinguish the strength of the multiplefrequencies at no less than 10,000 times a second.

It is thus another aspect of at least one embodiment of the invention toprovide for measurements that accurately capture the real-time dynamicprocess of combustion.

In an alternative embodiment, the sensor system may include a Fourierspectroscope positioned between the light source and the combustionchamber on the light guide. The Fourier spectroscope may measure andtime-modulate the multiple frequencies passing into the combustionchamber. A demodulating intensity detector may be positioned on thelight guide after the combustion chamber providing a time signalmeasuring a combination of the multiple frequencies and demodulating thetime signal to distinguish the strength of the multiple frequencies.

It is thus another aspect of at least one embodiment of the invention toprovide for a system that easily compensates for variation in thespectra of the exciting light signal.

The Fourier spectroscope may employ a photoelastic modulator to vary itseffective optical length.

It is thus another aspect of at least one embodiment of the invention toprovide a novel high speed Fourier spectroscope that can providesufficiently fast measurements for combustion gas analysis.

The plug may be a spark plug providing a spark for the internalcombustion engine.

It is thus another aspect of at least one embodiment of the invention toprovide for measurement in the vicinity of the spark in operating theinternal combustion engine.

These particular objects and advantages may apply to only someembodiments falling within the claims and thus do not define the scopeof the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a modified spark plug holding alight guide for receiving light to interact with combustion gases andtransmitting the light back to a spectroscope for high-speed analysis;

FIG. 2 is a block diagram of a spatial heterodyne spectrometer suitablefor use as the spectroscope of FIG. 1;

FIG. 3 is a diagram of the process steps of converting an image from thespatial heterodyne spectrometer into a spectrum and in performingsignature matching;

FIG. 4 is a block diagram of the alternative embodiment of the inventionusing the spark plug of FIG. 1 but employing a Fourier spectrometerupstream from the spark plug; and

FIG. 5 is a figure similar to that of FIG. 3 showing those steps ofsignal analysis in the embodiment of FIG. 4 that differ from theembodiment of FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to FIG. 1, a high-speed spectrographic sensor 10 of thepresent invention provides for a modified spark plug 12 that may be fitto a combustion chamber 18. In the manner of conventional spark plugs,the spark plug 12 may provide a conductive threaded flange 14 fittingwithin a corresponding threaded bore in the wall 16 of the combustionchamber 18, providing a seal therewith.

The spark plug 12 provides a center electrode 20 coaxially within aceramic insulator 22 and passing from outside of the chamber 18 where itis accessible at a high-voltage terminal 24 to inside the chamber 18where it extends out of the insulator 22 as an electrode tip 26. Aground electrode 28 extends from the flange 14 into the combustionchamber 18 to a point opposite the electrode tip 26 across a spark gap27 in a manner known in the art.

The insulator 22 or threaded flange 14 also holds a light guide 30passing through the insulator 22 or threaded flange 14 from outside thecombustion chamber 18 to a point within the combustion chamber 18 nearthe spark gap 27. The light guide 30 may be, in a preferred embodiment,two adjacent optical fibers 32 and 34, one for carrying light into thecombustion chamber 18 and one for carrying light out of the combustionchamber 18 for sensing.

The fiber 32 carrying the light into the combustion chamber 18 mayreceive light from a broad spectrum light source, such as anincandescent bulb in the form of a quartz tungsten-halogen lamp, or awideband LED or broadband laser, providing substantial energy in therange of 2000 nm to 3000 nm and preferably in a range of 2400 nm to 2600nm and having a known spectrum.

A mirror 36 is positioned across a gap 37 from the point where the lightguide 30 terminates in the combustion chamber 18. The mirror 36 ispositioned so that light passing through optical fiber 32 exits thelight guide 30 and passes across the gap 37 to strike the mirror 36, tobe reflected back across the gap 37 and be received by fiber 34. Theoptical path through the gap 37 may be as great as 10 mm to allow thelight to interact with combustion gases in the region of the electrodetip 26.

Light received from optical fiber 34, after interacting with thecombustion gases, passes through a filter 40, for example, a bandlimiting filter of the desired frequency range (e.g. 2400-2600 nm). Thefiltered light is then received by a spectrometer 42 which in thepreferred embodiment is a spatial heterodyne spectrometer.

The spectrometer 42 provides a digitized output 44 received by acomputer 46. The computer executes a program to display ahigh-resolution absorption spectrum 48 (based on known or measuredspectrum of light source 38) extracted every 100 μs and no less thanevery 1000 μs and consisting of hundreds of resolved frequency pointsand no less then twenty resolved frequency points. The computer 46,operating according to the stored program, may also identify aquantitative parameter value 49, being for example a temperature of thecombustion gases or a species mole fraction such as water concentrationor other similar measurement, as will be described.

Referring now to FIG. 2, the spatial heterodyne spectrometer 42 providesan open aperture and high-speed response made possible by its efficientuse of minor energy obtained through fiber 34. Spectrometers of thistype are described in U.S. Pat. No. 5,059,027, issuing Oct. 22, 1991,assigned to the assignee of the present invention, and herebyincorporated by reference. Such a spectrometer receives a light signal50 from the fiber 34 and collimates this light using an optical assembly52 to provide for a beam 53 having generally an aligned wavefront 54.

A dispersive optical system 56 tips the wavefronts 55 of each ofmultiple frequency component in the light signal 50 (only two shown) toan angle α dependent on the wavelength of that frequency component. Thewavefront-modified beam 58 is then received by an imaging opticalassembly 60 to project an image on a solid-state image detector 62 suchas an extended InGaAs line scan camera commercially available fromXenics Leuven, Belgium. The signal from the solid-state image detector62 may be digitized and sampled per block 63 to produce an image 64 atapproximately 1000 times per second or as much as 10,000 times persecond.

Referring now also to FIG. 3, the image 64 from the solid-state imagedetector 62 will contain a series of bands of different intensities 66caused by interference in the image produced by the constructive anddestructive interference of the wavefronts 55 as tipped by dispersiveoptical system 56. The information of this image 64 may be collapsed toa single dimension (x) to produce a spatially dependent signal 68 withimproved signal-to-noise ratio that better utilizes all of light energyfrom the fiber 34 both improving the speed and the resolving power ofthe spectrum.

This signal 68, when operated on by the Fourier transform 70, as may beimplemented in the computer 46 of FIG. 1, produces a high-resolutionspectrum 48 providing resolvable points for more than 100 differentfrequencies. The high-resolution spectrum 48 may be compared to spectrum74 of a library 76 of different signature spectra 74 by a correlator 78,where each signature spectra 74 is associated with a known physicalparameter that is to be extracted. For example, the multiple spectra 74may each represent measurements of combustion gases at a differenttemperature. Alternatively the multiple spectra 74 may each represent ameasurement of a different water concentration or another speciesconcentration.

The correlator 78 finds the best correlation between high-resolutionspectrum 48 and each of spectra 74 to output a measured temperature orother quantitative parameter value 49 as shown in FIG. 1, according tothe parameter associated with the most highly correlated spectra 74.

Referring now to FIG. 4, in an alternative embodiment the light source38 provides light to a filter 40 operating in a manner described abovewith respect to filter 40 in FIG. 1. The filtered light is then providedto a Fourier spectrometer 71. The Fourier spectrometer 71 operates in amanner similar to conventional Fourier spectrometers by separating thelight beam into two paths one of which is changed in effective length tocreate interference between the light of the two paths. The interferenceeffectively modulates by frequency each of the wavelengths of light fromthe light source 38 with that wavelength having highest frequency beingmodulated at the highest rate. A Fourier transform of this modulationreveals the spectrum of the light. Ideally the changing cavity length isa simple linear function, for example, following a triangle or sawtoothwave 75.

The output of the Fourier spectrometer 71 is thus a modulated light beamwhich is sent to the fiber 32 and which may be sampled locally at alocal sensor 73 to allow local characterization of the spectrum of thelight before modification by combustion gases as will be described. Themodulated light from the Fourier spectrometer 71 passes through thefiber 32 to the spark plug 12, as described above with respect to FIGS.1 and 2, and is modified by combustion gases and received by fiber 34ultimately to be provided to a sensor 72. Sensor 72 is not frequencydiscriminating and thus may employ an open aperture to efficientlymeasure multi-spectral light intensity. A Fourier transform of themodulated intensity at sensor 72 yields a spectrum which when comparedto the spectrum calculated from the sensor 73 provides an absorptionspectrum.

Referring still to FIG. 4, in order to provide the necessary speed andresolution for measuring combustion gases, the Fourier spectrometer 71differs from those spectrometers of the prior art by eliminating amechanically movable mirror or optical element that could not providesufficiently responsive modulation. Instead the Fourier spectrometer 17employs a non-mechanical cavity length control, for example, aphotoelastic modulator 77 to provide for a sweeping of the cavity lengthat least 1000 times per second and as much as 10,000 times per second.

Referring to FIG. 5, the sensor 73 used in conjunction with the highspeed Fourier spectrometer 71 thus produces a time signal 80 thatprovides a high resolution spectrum of more than 100 points at asampling rate as described above.

The system of the present invention may be employed while the internalcombustion engine is operating to measure gas temperatures in thevicinity of the electrode at an extremely high rate and accuracy. Forexample, it is beleived that a temporal resolution of 100 μs (˜1 deg.crank angle) with a better than 4 cm⁻¹ spectral resolution to provide atemperature resolution of ˜5 degrees C. or less than 0.1% to 1000K and1% to 3000K.

It is specifically intended that the present invention not be limited tothe embodiments and illustrations contained herein, but include modifiedforms of those embodiments including portions of the embodiments andcombinations of elements of different embodiments as come within thescope of the following claims.

1. A high-speed spectrographic sensor for internal combustion enginescomprising: a plug receivable into a combustion chamber of an operatinginternal combustion engine; a light source providing light at multiplefrequencies having wavelengths less than 3000 nm; a light guide held bythe plug and receiving the light source to communicate the light intothe combustion chamber for interaction with combustion gases and tocommunicate the light out of the combustion chamber for sensing; and asensor system including a slit-less spectroscope receiving light fromthe light guide, the slit-less spectroscope attached to the light guideto receive light from the light guide directly also without anintervening slit; the spectroscope distinguishing strength of no lessthan 20 multiple frequencies of the light at a rate of no less than 1000times per second after interaction with the combustion gases; whereinthe spectroscope provides an optical system spatially separatingmultiple light frequencies as received simultaneously from the lightguide after passage through the combustion chamber to permitsimultaneous monitoring of the multiple frequencies at different spatiallocations.
 2. The spectrographic sensor of claim 1 wherein the slit-lessspectrometer is selected from the group consisting of: a spatialheterodyne spectroscope and a Fourier spectrometer, the spectrometerreceiving the light from the light guide after the light has passedthrough the combustion chamber.
 3. The spectrographic sensor of claim 1wherein the sensor system further includes a computer deducing andoutputting temperature within the combustion chamber from the strengthsof the multiple frequencies.
 4. The spectrographic sensor of claim 1wherein the sensor system further includes a computer deducing andoutputting water concentration within the combustion chamber from thestrengths of the multiple frequencies.
 5. The spectrographic sensor ofclaim 1 further including a computer deducing a physical parameter ofcombustion gases by matching the strength of the multiple frequencies tocorresponding multiple frequencies of signatures representing knowndifferent physical parameters within the combustion chamber from thestrengths of the multiple frequencies.
 6. The spectrographic sensor ofclaim 1 wherein the light source provides frequencies having wavelengthssubstantially within a range of 2400-2600 nm.
 7. The spectrographicsensor of claim 1 wherein the sensor system distinguishes the strengthof no less than 100 multiple frequencies.
 8. The spectrographic sensorof claim 1 wherein the sensor system distinguishes the strength of themultiple frequencies no less than 10,000 times a second.
 9. Thespectrographic sensor of claim 1 wherein the sensor system includes: aFourier spectroscope positioned between the light source and thecombustion chamber on the light guide measuring and time-modulating themultiple frequencies; and a demodulating intensity detector positionedon the light guide after the combustion chamber providing a time signalmeasuring a combination of the multiple frequencies and demodulating thetime signal to distinguish the strength of the multiple frequencies. 10.The spectrographic sensor of claim 9 wherein the Fourier spectroscopeemploys a photoelastic modulator to vary an effective optical length.11. The spectrographic sensor of claim 1 wherein the plug is a sparkplug providing a spark for the internal combustion engine.
 12. A methodof high-speed spectrographic sensing of combustion gases in an internalcombustion engine comprising: (a) placing a plug in a combustion chamberof an operating internal combustion engine, the plug providing a lightguide leading to within the combustion chamber; (b) introducing light atmultiple frequencies having wavelengths less than 3000 nm into the lightguide to interact with combustion gases; and (c) receiving light fromthe plug through the light guide at a slit-less spectroscope attached tothe light guide to receive light from the light guide directly alsowithout an intervening slit; (d) spatially separating multiple lightfrequencies as received simultaneously by an optical system of thespectroscope after passage through the combustion chamber to permitsimultaneous monitoring of the multiple frequencies at different spatiallocations; the spectroscope distinguishing strength of no less than 20multiple frequencies of the light at a rate of no less than 1000 timesper second after interaction with the combustion gases.
 13. The methodof claim 12 wherein the determining spatially heterodynes the lightreceived after interaction with the combustion gases to determineattenuation of the multiple frequencies.
 14. The method of claim 12further including outputting temperature within the combustion chamberfrom the strengths of the multiple frequencies.
 15. The method of claim12 further including outputting water concentration within thecombustion chamber from the strengths of the multiple frequencies. 16.The method of claim 12 further including matching the strength of themultiple frequencies to multiple signatures with similar multiplefrequencies and representing known different physical parameters todeduce a physical parameter within the combustion chamber.
 17. Themethod of claim 12 wherein the light includes frequencies havingwavelengths substantially within a range of 2400-2600 nm.
 18. The methodof claim 12 wherein the attenuation of no less than 100 multiplefrequencies is determined.
 19. The method of claim 12 wherein theattenuation of the multiple frequencies is determined no less than10,000 times a second.
 20. The method of claim 12 further includingmeasuring and time-modulating the multiple frequencies before the lightinteracts with the combustion gases; and demodulating a time signalderived from a strength of the multiple frequencies to distinguish thestrength among the multiple frequencies after interaction with thecombustion gases.
 21. The method of claim 12 further including providingan ignition spark in the vicinity of the light guide within thecombustion chamber.